Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2001-02-27
2003-05-13
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044320, C369S094000
Reexamination Certificate
active
06563780
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Application No. 00-10875 filed Mar. 4, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup for high-density recording/reproduction compatible for optical discs having different formats, and more particularly, to an optical pickup for high-density recording/reproduction adopting a single objective lens, which is compatible for optical discs having different formats, and corrects for chromatic aberration caused by different wavelengths of light and/or spherical aberration due to differences in thickness of optical discs.
2. Description of the Related Art
Optical pickups are a device for recording information on or reproducing information from an optical disc by focusing a laser beam on the optical disc with an objective lens. The recording and reproduction capacity is determined by the size of a focused spot. The size of the focused spot is related with a wavelength (&lgr;) of the laser beam, and a numerical aperture (NA) of the objective lens, as shown in equation (1):
size of focused spot
∝ &lgr;/NA (1)
For a higher recording density of about 15 gigabytes or more, the size of the spot being focused on the optical disc must be further reduced. To form a small spot for high-density recording, as can be inferred from equation (1), it is essential to adopt a blue laser as a light source, which emits light having a short wavelength of about 410 nm, and an objective lens having an NA of 0.6 or more.
On the other hand, coma W
31
, which occurs due to tilting of the optical disc, is associated with a tilt angle (&thgr;) of the information recording surface of the disc with respect to an optical axis, a refractive index (n) of the disc substrate, the thickness (d) of the disc substrate, and the NA of the objective lens, as expressed by equation (2):
W
31
=
-
d
2
·
n
2
(
n
2
-
1
)
sin
θcos
θ
(
n
2
-
sin
2
θ
)
5
/
2
NA
3
(
2
)
To ensure tolerance with respect to the tilt of disc for high density recording, there is a tendency of reducing the thickness (d) of the disc substrate. For example, compact discs (CDs) have a thickness of 1.2 mm and digital versatile discs (DVDs) have a thickness of 0.6 mm. Also, there is a high possibility that the thickness of future generation DVD family media (so-called high-definition (HD)-DVDs), which are recently being developed, is determined to be 0.6 mm or less.
Optical pickups for high-density recording/reproduction in/from future generation DVDs adopt a light source which emits a blue laser beam, and an objective lens optimized to be suitable for the blue laser beam and the thickness of a future generation DVD substrate.
For compatibility with existing discs, such as DVDs, the optical pickup for high-density recording and reproduction needs another light source which emits a red laser beam. The reason why both blue and red light sources are adopted in the optical pickup for future generation DVDs is for compatibility with DVD-recordable (DVD-R) and multi-layered DVDs, which have a low reflectivity with respect to blue light.
The objective lens of the optical pickup for future generation DVDs is designed to be suitable for blue light and the thickness of a future generation DVD substrate. Thus, when a DVD is adopted as a recording medium, a red light spot focused on the recording surface of the DVD by the objective lens includes chromatic aberration due to a difference in wavelengths of red and blue light. In addition, when the thickness of a future generation DVD substrate used is different from that of a DVD substrate, spherical aberration caused by the thickness difference of the discs occurs.
FIG. 1
illustrates the optical path difference (OPD
rms
) in an optical pickup adopting an objective lens designed exclusively for 405 nm light with respect to wavelength variations of light incident on the objective lens. In
FIG. 1
, OPD
rms
refers to the amount of aberration in a light spot focused by the objective lens and is expressed in wavelengths.
As shown in
FIG. 1
, when 405 nm light is incident on the objective lens, almost no aberration occurs, so that the OPD
rms
at 405 nm is close to zero. In contrast, when 650 nm light is incident on the objective lens, the OPD
rms
at a wavelength of 650 nm becomes 0.15&lgr; due to increased aberration.
Thus, in consideration of a standard aberration allowance, i.e., OPD
rms
=0.07&lgr;, in the related field, the optical pickup for future generation DVDs, which is designed to focus 650 nm light with the objective lens optimized for 405 nm light, is not compatible with DVDs. In other words, for the compatibility with DVDs, the optical pickup for future generation DVDs needs to correct for chromatic and/or spherical aberration mentioned previously.
Referring to
FIG. 2
, a conventional aberration correcting apparatus includes an objective lens
3
which focuses an incident beam and a condensing lens
5
which further condenses the beam focused by the objective lens
3
to form a light spot on an optical disc
1
. As shown in
FIG. 3
, a distance between the condensing lens
5
and the objective lens
3
is adjusted according to thickness variations (&Dgr;d) of the optical disc substrate
1
and wavelength variations of light used, such that aberration is corrected.
However, as for such a conventional aberration correcting apparatus adopting two lenses, the condensing lens
5
and the objective lens
3
, assembling the two lenses is complicated. Also, the objective lens
3
and the condensing lens
5
must be actuated for both tracking and focusing control, and adjusting the distance between the objective lens
3
and the condensing lens
5
, so the structure of the entire actuator becomes complicated.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide an optical pickup for high-density recording/reproduction adopting a single objective lens, in which chromatic aberration caused by different wavelengths of light and/or spherical aberration due to thickness variations of optical discs is corrected, and thus the optical pickup is compatible for optical discs having different formats.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above object and other objects of the present invention, there is provided an optical pickup compatible for optical discs having different formats. A first light source emits a beam having a relatively short wavelength suitable for a first optical disc with a format and a second light source emits a beam having a relatively long wavelength suitable for a second optical disc with another format. An objective lens, designed to be suitable for the first optical disc and the wavelength of the beam emitted from the first light source, focuses an incident beam to form a light spot on a corresponding optical disc. An optical path changing system alters a traveling path of the beams emitted from the first and second light sources. A first photodetector receives and photoelectrically converts the beam reflected from a corresponding optical disc and passed through the objective lens after having been emitted from the first light source. A first signal processing unit detects a reproduction signal of the first optical disc from the signal output from the first photodetector. A first light splitter splits an incident beam into at least two beams including first and second beams to be focused as a main light spot and a sub-light spot, respectively, on the second optical disc after having been emitted from the second light source. A second photodetector comprises first and second light receiving portions which respectively receive and
Ahn Young-man
Chung Chong-sam
Kim Tae-kyung
Suh Hea-jung
Edun Muhammad
Staas & Halsey , LLP
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